1. Field of the Invention
The present invention relates to a photomask technique used in micromachining of a semiconductor integrated circuit, a charge-coupled device (CCD), a color filter for a liquid crystal display (LCD) element, a magnetic head or the like.
2. Description of the Related Art
In recent years, in order to meet the demand for miniaturization of circuit patterns involved with the increasing packaging density of large-scale integrated circuits, advanced semiconductor micromachining techniques have become extremely important. For example, increasing the packaging density of a large-scale integrated circuit essentially requires a technique of thinning wires of wiring patterns in the circuit or a technique of miniaturizing a contact hole pattern for interlevel wiring of a cell. The trend toward miniaturization of circuit patterns of large-scale integrated circuits is being accelerated because it is the most effective approach to increase the operation speed thereof and reduce the power consumption thereof.
Most of such advanced micromachining is based on the photolithography technique using a photomask. Therefore, the photomask, as well as the exposure apparatus and the resist material, is a fundamental technique for miniaturization. Therefore, in order to provide a photomask having a wiring pattern of thinned wires and a miniaturized contact hole pattern described above, development of a technique for forming a finer and more precise pattern on a photomask blank has been pursued.
To form a highly precise photomask pattern on a photomask substrate, it is essential that a resist pattern formed on a photomask blank is highly precise. When micromachining a semiconductor substrate, reduction projection photolithography is performed, and therefore, the size of the pattern formed on the photomask is about four times as large as the size of the pattern formed on the semiconductor substrate. However, this does not mean that the restriction on the precision of the pattern formed on the photomask is relaxed. On the contrary, the photomask pattern has to be formed with higher precision than the pattern provided on the semiconductor substrate after exposure.
Furthermore, at present, circuit patterns written on semiconductor substrates by photolithography are significantly small compared with wavelengths of exposure light. Thus, if a photomask that has a photomask pattern formed from a circuit pattern on a semiconductor substrate through 4-fold magnification is used to perform reduction projection exposure, the photomask pattern cannot be accurately transferred onto the resist film due to interference of the exposure light or the like.
Thus, as a super-resolution mask, there are commonly used an OPC mask that corrects the optical proximity effect, which degrades the transfer characteristics, by application of the so-called optical effect correction (OPC) and a phase shift mask that makes the phases of adjacent aperture patterns different by 180°, thereby making the optical amplitude at the middle of the adjacent aperture patterns zero. For example, on the OPC mask, an OPC pattern (a hammer head, an assist bar or the like) that has a size one-half or less the size of the circuit pattern has to be formed.
As described above, not only the photolithography for forming a circuit pattern on a semiconductor substrate but also the photolithography for forming a pattern on a photomask blank require a highly precise patterning technique. One of indications of the performance of the photolithography technique is the “limiting resolution”. The photolithography used in patterning of a photomask is required to have a limiting resolution equal to or higher than that of the photolithography used in circuit patterning on a semiconductor circuit.
Typically, when forming a photomask pattern, a photoresist film is formed on a photomask blank having a light-shielding layer on a transparent substrate, the photoresist film is irradiated with an electron beam to write a pattern thereon, and the photoresist film is developed to provide a resist pattern. Then, using the resist pattern as an etching mask for the light-shielding layer, the light-shielding layer is patterned to form a photomask pattern. In order to form a fine photomask pattern by such an approach, it is important to make the photoresist film thinner and to select an appropriate material for the light-shielding layer.
As the photomask pattern to be formed becomes finer, the resist pattern becomes finer. However, if the resist pattern is miniaturized without reducing the thickness of the resist film, the aspect ratio (that is, the ratio between the resist thickness and the pattern width) of the part of the resist serving as the etching mask for the light-shielding layer increases.
In general, as the aspect ratio of the resist pattern increases, the pattern becomes more susceptible to degradation, and if the resist pattern is used as the etching mask, the precision of the transfer thereof onto the light-shielding layer decreases. In an extreme case, a part of the resist pattern may fall or peel off, resulting in a defective pattern. Thus, as the photomask pattern becomes finer, the thickness of the resist film used as the etching mask for the light-shielding layer has to be reduced to prevent the aspect ratio thereof from being undesirably great.
On the other hand, as for the material of the light-shielding layer that is to be patterned using the photoresist as an etching mask, many materials have already been proposed. Among others, chromium compound can contain a large amount of information, so that the chromium compound has been practically always used as the material of the light-shielding film, and forming the light-shielding film from the chromium compound has been substantially established as a standard process step. For example, in Japanese Patent Laid-Open Nos. 2003-195479 and 2003-195483 and Registered Japanese Utility Model No. 3093632, there has been disclosed a structure of a photomask blank in which a light-shielding film having a light-shielding capability, which is required for a photomask blank designed for ArF exposure, is made of chromium compound.
In general, the light-shielding film made of chromium compound is patterned by oxygen-and-chlorine-based dry etching. However, such etching often has a significant effect on an organic film, such as the photoresist. Thus, if the light-shielding film is etched using a relatively thin resist film as a mask, the resist is damaged during the etching, the configuration of the resist pattern changes, and it is difficult to accurately transfer the original resist pattern onto the light-shielding film.
However, it is technically difficult to make the photoresist, which is an organic film, have both high resolution and patterning precision and high etching resistance. As far as a conventional patterning process is used, there is a tradeoff between the resolution and the etching resistance. Specifically, the photoresist film has to be made thinner to achieve higher resolution, while thinning of the photoresist film has to be limited to assure the etching resistance during the patterning step.
Thus, in order to form a highly precise photomask pattern while reducing the burden on a photoresist, it is necessary to propose a novel structure of a photomask blank by selecting an optimal material for a light-shielding film.